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Solar systems out there very like our own

Astronomers have discovered four planets orbiting two stars similar to Earth's sun, raising hopes that other life may exist in the universe.

However, the stars are 28 and 84 light years away - placing them far beyond the reach of spacecraft.

The first three planets orbit the star 61 Virginis which can be seen with the naked eye in the constellation of Virgo.

They have the greatest potential for extraterrestrial life with the star being described as "virtually a twin of the Sun" and the planets' masses ranging from 5.3 to 24.9 Earth masses.

Professor Chris Tinney, from the University of New South Wales, said: "These planets are particularly exciting. Neptune in our Solar System has a mass 17 times that of the Earth.

"It looks like there may be many Sun-like stars nearby with planets of that mass or less.

"They point the way to even smaller planets that could be rocky and suitable for life."

Further afield, the fourth planet discovered is similar to Jupiter in mass and is orbiting the Sun-like star, 23 Librae, in the constellation of Libra.

It has a 14-year orbit, compared with Jupiter's 12-year orbit and is the second planet found around this star since 2006.

Fellow scientists Dr Simon O'Toole of the Anglo-Australian Observatory, said: "In fact, what we detect from this star system is very like what we'd detect from our own Solar System if we were observing it from a distance, because Jupiter has the strongest gravitational effect of all our Sun's planets."

Hugh Jones, from the University of Hertfordshire, said: "We are now in a position to quantify how common planets like Jupiter are around stars like our Sun.

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"Compared to the Solar System, most extrasolar systems look odd, with planets in very small or very elliptical orbits.

"In contrast, this new planet has an orbit that is both large, and nearly circular - and, for the first time, we are beginning to see systems that resemble our own."

The discoveries were made by an international team of planet hunters using the Anglo-Australian Telescope in New South Wales and the Keck Telescope in Hawaii.

They used the "Doppler wobble" technique, which measures how stars are tugged around by their planets' gravity.

Carnegie Institute of Washington's Dr Paul Butler said: "These detections are truly at the current state-of-the-art.

"The inner planet of the 61 Virginis system is among the two or three lowest-amplitude planetary signals that have been identified with confidence.

"We've found there's a tremendous advantage to be gained from combining data from two world-class observatories, and it's clear that we'll have an excellent shot at identifying potentially habitable planets around the very nearest stars within just a few years."